This invention relates to a process of forming a metal or metal compound coating on a face of a heated glass substrate during its conveyance in a given direction through a coating station at which the said face is contacted with at least one stream of droplets comprising a substance or substances from which said coating metal or metal compound is formed on said face, such stream(s) being directed at a downward inclination to such face.
Such a process can be employed for coating glass sheets and for coating a continuously freshly formed ribbon of flat glass. The process can be used for forming coatings, e.g. metal oxide coatings, which modify the apparent colour of the glass and/or which have some other required properties in respect of incident radiation, e.g. an infra-red-reflecting property.
A problem which arises in the performance of processes of the kind referred to is that of achieving coatings of homogeneous structure and providing uniform coverage of the substrate. In United Kingdom Pat. No. 1,516,032 a process of the kind referred to is described wherein the formation of an homogeneous coating is promoted by discharging the coating material against the substrate as a stream which is inclined downwardly towards the substrate, in the direction of its conveyance, so that the acute or mean acute angle of incidence of such stream on the substrate, measured in a plane normal to the substrate and parallel with its direction of conveyance, is not more than 60.degree..
United Kingdom Pat. No. 1,523,991 discloses a glass coating process of the same kind in which, for the same purpose of promoting the homogeneity and uniformity of the coating, suction forces are created in exhaust ducting situated so as to cause gases environmental to the droplet stream to flow away from said stream and into such ducting, substantially without affecting the paths of the droplets towards the substrate.
Even when observing the conditions proposed in the said prior patents, defects have sometime occurred below or at the surface of the coating which defects although often not very prominent, nevertheless disqualify the product from the top quality ratings which are now in demand. If the defects are at the surface of the coating, the quality of the product can in some but not all cases be improved by an after-coating surfacing treatment, but of course such additional treatments add to the product cost.
It appears that residual defects occurring notwithstanding the previously proposed remedial measures are at least in part attributable to deposition of substances forming in the environment above the substrate, or to impairment of the coating during its formation by contact with a coating precursor compound which has not reacted or has not completely reacted at the place where the droplet stream contacts the heated substrate. Attempts have therefore been made to reduce the occurrence of such defects by careful control of suction forces responsible for the withdrawal of gases from the coating station. Improvements have also been sought by modifying the number and the arrangement of the exhaust ducts. Such measures are useful but do not always enable the required results to be achieved.